“They were children of the 1930s. When World War Two was happening they wanted to help the Russians beat the Nazis,” Robert Meeropol, 69, told The Telegraph.

Michael and Robert Meeropol at the White HouseCredit:
Alan Heath

There is no dispute that Julius Rosenberg was a Soviet agent; he even had a codename, “Liberal”. The brothers accept he was involved in recruitment and industrial espionage and therefore guilty of conspiracy.

But they dispute that he was involved in passing the atomic secrets which led to his execution, citing declassified Soviet documents showing that he had been de-activated as an agent several months before the crucial material fell into Russian hands.

The focus of their campaign has always been on the involvement of their mother. “It was almost as if Ethel had disappeared into Julius,” Mr Meeropol said.

At the trial Greenglass, who received a 10 year jail term for spying, said Ethel Rosenberg played a key role in recruiting him and even transcribed classified information on a typewriter at a meeting held at the Rosenbergs’ Lower East Side Manhattan apartment in September 1945.

The brothers have always insisted the meeting never took place and that Greenglass, who died in October 2014, lied implicating his sister in return for immunity for his wife – who was never charged.

The only known picture of Ethel Greenglass Rosenberg and her brother, David Greenglass. The pair stand outside their childhood home, in this undated photo probably taken during World War IICredit:
AP

They point to evidence Greenglass gave the Grand Jury, which was released last year. In it, he explicitly denied speaking to his sister about spying.

Robert Meeropol, an activist, author and lawyer, was only three when his parents were arrested, He remembers visiting them in Sing Sing and recalls the week of the execution three years later, when hopes of a reprieve were finally dashed.

“My parents created a very calm and normal atmosphere,” he said. “They wanted to fool me and I wanted to be fooled.”

Thanks to the Meeropols, Robert and his older brother had a normal upbringing. “We dropped from public sight. I grew up in blissful ignorance. Nobody knew who I was. The only ones who did were sympathetic.”

The search for the truth started in the 1970s when they sued an author for publishing the Rosenbergs’ prison letters without permission. They then used freedom of information legislation to extract more information.

Ethel and Julius Rosenberg's young sons, Michael (left) and Robert (right) visit their parents at Sing Sing accompanied by Emanuel Block, the Rosenbergs defense attorneyCredit:
AP

Before Ethel’s arrest, an assistant attorney general told the FBI that there was “insufficient evidence” to charge her, but that she could still be “used as a lever against her husband.”

“We wanted the facts out in public. The case was 20 years old. The fear of going public had gone,” Mr Meeropol said.

“We had a very strong belief in our parents’ innocence. We were going to ask for all of the material, even if it hurt our position,” Mr Meeroopol recalled.

Robert Meeropol, the son of Ethel Rosenberg who wants Obama to exonerate her of espionage, which led to her execution in 1953Credit:
David Millward

“As the material came out, it became a double-edged sword. It showed Julius had been involved in something. Ethel had been involved in nothing.”

The case for Ethel Rosenberg’s exoneration has been backed by the City of New York which issued a proclamation last year on what would have been her hundredth birthday, declaring her innocence.

Even if the outgoing president does accede to their request, the triumph could prove pyrrhic, Mr Meeropol admitted.

“Given that Cohn was his mentor, I would not put it past Trump, if Obama did something, to try and reverse it.”

But the fight has been worthwhile and, in the current climate, has relevance today, he added.

“Using the judicial process against opponents is what totalitarian governments do.”